1. Field of the Invention
The invention relates to a heat roller device and especially to heat roller device for use in a heat roller system which is used in an electrophotographic copier, a laser printer, a fax machine, and the like, to fix a toner image.
2. Description of Related Art
Conventionally, in an electrophotographic copier or the like, a heat roller system is widely used to thermally fix a toner image formed on recording material. Typically, the unfixed toner image, positioned on the recording material, is fixed on the recording material by one pass of the recording material between a heat roller and press roller located opposite it.
In a conventional fixing device of the heat roller type, a heat roller includes a hollow metal tube with an offset prevention layer consisting of fluororesin, or the like, formed on its outer surface and a heat lamp, such as a halogen lamp or the like, positioned in the interior of the tube, as disclosed in published utility model JP HEI 3-45248 or JP patent disclosure document HEI 5-19659.
Furthermore, JP patent disclosure document SHO 55-72390 discloses a process in which, instead of a heating lamp being used as the heat means of the heat roller, the surface of cylindrical insulating material is provided with a heat generating resistance body so that the heat roller itself produces heat by turning on this heat generating resistance body.
In above-described fixing devices, first the heat roller is heated by supplying power to the heat lamp or heat generating resistance body to raise the temperature of its outer surface up to a fixing temperature (for example, up to 180.degree. C.), which is also referred to as "preheating". After the outer surface of the heat roller has reached the fixing temperature, the surface temperature of the heat roller is regulated. This regulated surface temperature is called the setting temperature while this operation is called "stand-by". This is done by regulating the power supplied to the heat lamp or the heat generating resistance body by means of a signal from a temperature sensor which determines the surface temperature of the heat roller. By one pass of the recording material between the temperature-controlled heat roller and the press roller, hereinafter referred to as "fixing work", the not yet fixed toner is heated as it is squeezed and thus the toner image is fixed on the recording material.
In an electrophotographic copier and the like, the recording material may be subjected to fixing work without interruption, with a passage width which is smaller (for example, transverse width of a B5 form) than the maximum passage width provided for the device (for example, transverse width of A3 form). In this case, the entire area of the maximum passage width of the heat roller is heated by the heat lamp or the heat generating resistance body and is also temperature controlled. If the recording material and the heat roller do not come into contact with one another, hereinafter referred to as the "non-paper transport area", to the same degree, an abnormal temperature increase occurs, to a slight extent, in which less heat is removed from it by the recording material compared to the part in which the recording material and the heat roller come into contact with one another, hereinafter referred to as the "paper transport area".
In the case in which, immediately following the above-described state, recording material with a large passage width, for example, an A3 form, is subjected to fixing work, scattering of the fixing property of the toner occurs due to the phenomenon of high temperature offset, or the like, which is caused by overmelting of the toner in the part with a high temperature as a result of the nonuniform temperature distribution of the heat roller.
Also, the excess temperature increase of the non-paper transport area in the longitudinal directions of the heat roller and the press roller causes thermal stress by which the durability of the heat roller and the press roller is highly adversely affected. The heating of the non-paper transport area also results in the excess power consumption.
To improve this situation, a process was devised in which the longitudinal direction of the heat roller surrounding the heat generation area of the heat lamp or heat generating resistance body is divided into at least two parts. Thus the width of the heat generating area is switched according to the passage width of the recording material, as disclosed, for example in JP patent HEI 3-1666 or JP patent disclosure document SHO 59-197067 and the like.
In the above described conventional process in which, the heat generating width in the longitudinal direction of the heat roller is switched according to the respective passage width of the recording material, it is, however, necessary to increase the number of areas to be divided accordingly as the number of different passage widths increases. If the number of divisions of the heat generating area is increased, both the number of electrodes for supplying the heat lamp or the heat generating resistance body as well as the number of switching elements for switching the heat generation width increase accordingly, causing the device to become complex and thus increasing costs.
Especially in the method using the heat lamp, the number of heat lamps having heat generating areas with different widths increase. Thus, it is necessary to make room available for arrangement of these several heat lamps in the interior of the heat roller. Therefore the diameter of the heat roller cannot be reduced; this, together with the increasing number of the above described components, prevents the device from being made smaller and compact.
As is described above, in a conventional heat fixing device, with the method in which, according to the passage width of the recording material, the heat generating width is switched in order to correspond to recording materials with different passage widths, the disadvantages include a complicated configuration, high costs and difficulty of reducing the size of the device and making it compact.